Spinning out of focus: The challenge of rotational line broadening in exoplanet reflection spectroscopy

¹ European Southern Observatory, Karl-Schwarzschildstrasse 2, 85748 Garching bei München, Germany
² Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
³ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany
⁴ Exzellenzcluster Origins, Boltzmannstraße 2, 85748 Garching, Germany
⁵ Institut für Astrophysik und Geophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁶ Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
⁷ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁸ Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁹ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
¹⁰ Observatorio de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Almería, Spain
¹¹ Centro de Astrobiología (CSIC-INTA), Camino bajo del castillo s/n, ESAC Campus, 28692 Villanueva de la Cañada, Madrid, Spain
¹² Thüringer Landssternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹³ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
¹⁴ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento, 1, 90134 Palermo, Italy
¹⁵ Departamento de Física de la Tierra y Astrofísica, Facultad de Ciencias Físicas, e IPARCOS-UCM (Instituto de Física de Partículas y del Cosmos de la UCM), Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁶ Institut de Ciéncies de l’Espai (CSIC-IEEC), Campus UAB, c/de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
¹⁷ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
¹⁸ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 18 December 2025
Accepted: 27 April 2026

Abstract

Context. Detecting light reflected off the dayside of an exoplanet in high-resolution spectroscopic data has proved to be a notoriously difficult endeavour. Despite several attempts, the faint signal has yet to be detected.

Aims. We present a new effort at finding reflection signatures and show how a strong rotational broadening of the reflected spectrum can complicate this objective.

Methods. We introduce a new figure of merit that quantifies the favourability of different systems for a reflection study, the reflection spectroscopy metric. Applying this metric, we identify the KELT-9 system, which features a highly misaligned, rapidly rotating host star, as the target for a case study based on a spectroscopic time series obtained by CARMENES. We also perform an injection-recovery test to determine the detectability of the signal in our data and demonstrate its sensitivity to rotational line broadening.

Results. The search for a genuine reflection signal in our data resulted in a non-detection. The injection-recovery test puts this finding into context by revealing the critical importance of taking rotational broadening into account when dealing with systems featuring rapidly rotating stars and large spin-orbit misalignments.

Conclusions. The case study presented here underscores the need to incorporate stellar rotation and spin-orbit misalignment into assessments of a given planet’s favourability to reflection studies.